Six underground storage tanks at the Hanford Nuclear Reservation along the Columbia River in Washington state were recently found to be leaking radioactive waste, but there is no immediate risk to human health, state and federal officials said.

The seeping waste adds to decades of soil contamination caused by leaking storage tanks at Hanford in the past and threatens to further taint groundwater below the site, but poses no near-term danger of polluting the Columbia River, officials said.

The newly discovered leaks were revealed by Governor Jay Inslee a week after the US Energy Department disclosed that radioactive waste was found to be escaping from one tank at Hanford.

Inslee said he was informed on Friday by outgoing US Energy Secretary Steven Chu that a total of six of the aging, single-walled tanks were leaking radioactive waste.

‘‘There is no immediate or near-term health risk associated with these newly discovered leaks, which are more than 5 miles (8km) from the Columbia River,’’ Inslee said in a statement released by his office.

‘‘But nonetheless this is disturbing news for all Washingtonians.’’

The governor said Chu told him that his department initially missed the other five leaking tanks because staff there did not adequately analyse data.

‘‘This certainly raises serious questions about the integrity of all 149 single-shell tanks with radioactive liquid and sludge at Hanford,’’ he said.

The Energy Department issued a brief statement acknowledging that six waste tanks were found to be leaking and adding that there was ‘‘no immediate public health risk’’.

Four of the tanks in question, including the two biggest of the group, are known to have leaked waste in the past as well, Suzanne Dahl, the tank waste treatment manager for the state Department of Ecology, said.

‘‘It points to the age of the tanks and how there’s going to be an increased probability of this happening in the future,’’ she said.

‘‘When waste is in the tanks, it’s manageable. Once it’s out of the tanks and in the soil, it’s much harder to manage it, remove it, and down the road you’re adding to contamination in the groundwater that already exists.’’

DECLINING LIQUID LEVELS

The Energy Department said a week ago that declining liquid levels in one tank at Hanford showed it was leaking at a rate of 568 litres to 1136 litres per year.

It subsequently informed state officials that a second, larger tank was leaking at about the same rate, while the four smaller tanks were leaking at a rate of about 56 litres per year, Dahl said.

The Department of Energy said last week that monitoring wells have identified no significant changes in concentrations of chemicals or radionuclides in the soil.

The two biggest tanks at issue have capacities of about 2,839,000 litres and 1,892,000 litres, while the four others are designed to hold up to 208,000 litres, Dahl said. All were constructed many decades ago.

The 1518-square-km Hanford Nuclear Reservation was established near the town of Hanford in 1943 as part of the Manhattan Project, the US government programme that developed the first atomic bombs.

Production of plutonium materials at the site continued through the Cold War and ended there in 1989 as work shifted to cleanup of nuclear and chemical waste at Hanford, considered one of the largest and most complex such projects in the country.

Weapons production at the site resulted in more than 43 million cubic metress of radioactive waste and 130 million cubic metres of contaminated soil and debris, according to the US Environmental Protection Agency, which said that about 1798 billion litres of contaminated water have been discharged into the soil.

As part of the cleanup, as much remaining liquid waste as possible was pumped out of the older single-shell tanks into sturdier double-walled tanks in a process completed in 2005, Dahl said.

But sludge, mud-like waste and pockets of liquid remained behind in the older tanks, and it is that material that was found to be seeping in the soil again from six tanks, she said. According to the DOE, one of those tanks currently holds about 1,692,000 litres of radioactive sludge.

Under the multibillion-dollar cleanup plan, the waste from the storage tanks will eventually be processed in a special treatment plant that will immobilise the waste in a glass-like material that can be safely disposed of underground in stainless steel canisters.

But Dahl said construction of the waste treatment plant was still years away.